Purchasing real security cameras can get expensive, but it is the best way to record everything that's going on in case you need the evidence for later. On the other hand if you just want to scare people away by making them think you have a security camera, or if all you need is a prop camera for some reason, then making a fake security camera is easily done and it only takes some cardboard and a few odds and ends.
Cut your cardboard into a rectangular tube that's between 6 and 8 inches long. Hot glue the edges together so that the cardboard tube is now one piece. Trace the empty ends of the tube and cut out more cardboard, then glue the two rectangles to the ends. You should now have a plain, cardboard box with no openings. Cut off any strings of hot glue that are dangling.
Cut the cardboard tube (a toilet paper tube will work just fine) until it's roughly an inch or two in length. You then need to glue the tube to one of the traced ends of the box you made. So, it you lay the rectangular box down longways you should have a small, round tube sticking off one end. It should resemble the body of a security camera, and the lens sticking out. Spray paint the entire thing black, using at least two coats to give it a solid, glossy look.
Glue your lens into the small, round tube. There should be no glue showing, and it should look like a camera. All you need to do is tack it up on the ceiling or high on a wall, and it will give the impression, from a distance anyway, that there's a security camera watching.
Feel free to add additional details. A small, red LED light on the camera is a nice touch, but so is running wire along one side as if it was a power cable.